The present invention is directed towards a superabsorbent polymer. A superabsorbent polymer is a crosslinked partially neutralized polymer, including crosslinked polyacrylic acids or crosslinked starch-acrylic acid graft polymers, that is capable of absorbing large amounts of aqueous liquids and body fluids, such as urine or blood, with swelling and the formation of hydrogels, and of retaining the aqueous liquids under a certain pressure in accordance with the general definition of superabsorbent polymer. Superabsorbent polymer may be formed into particles, generally referred to as particulate superabsorbent polymer, wherein the particulate superabsorbent polymer may be post-treated with surface crosslinking, surface treatment, and other treatment to form particulate superabsorbent polymer compositions. The acronym SAP may be used in place of superabsorbent polymer, superabsorbent polymer composition, and particles hereof. A primary use of superabsorbent polymer and superabsorbent polymer compositions is in sanitary articles, such as babies' diapers, incontinence products, or sanitary towels. A comprehensive survey of superabsorbent polymers, and their use and manufacture, is given in F. L. Buchholz and A. T. Graham (editors) in “Modem Superabsorbent Polymer Technology,” Wiley-VCR, New York, 1998.
Superabsorbent polymers may be prepared by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like in the presence of relatively small amounts of an internal crosslinker such as a di- or poly-functional monomers that may include N,N′-methylenebisacrylamide, trimethylolpropane triacrylate, ethylene glycol di(meth)acrylate, or triallylamine. The di- or poly-functional monomer materials may serve as covalent internal crosslinking agents to lightly crosslink the polymer chains, thereby rendering them water-insoluble, yet water-swellable. These lightly crosslinked superabsorbent polymers contain a multiplicity of carboxyl groups attached to the polymer backbone. These carboxyl groups generate an osmotic driving force for the absorption of body fluids by the crosslinked polymer network.
In addition to covalent internal crosslinking agents, ionic internal crosslinking agents have been utilized to prepare superabsorbent polymers as well. The ionic internal crosslinking agents are generally coordination compounds comprising polyvalent metal cations, such as Al3+ and Ca2+, as disclosed in U.S. Pat. No. 6,716,929 and U.S. Pat. No. 7,285,614. The superabsorbent polymers disclosed in these patents have a slow rate of absorption, due to the presence of ionic crosslinks. In this context, slow rate may be measured by the Vortex Test and slow rate SAPs generally have a vortex time of 180 sec or more.
Superabsorbent polymers, useful as absorbents in absorbent articles such as disposable diapers, need to have adequately high sorption capacity, as well as adequately high gel strength. Sorption capacity needs to be sufficiently high to enable the absorbent polymer to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Gel strength relates to the tendency of the swollen polymer particles to deform under an applied stress, and needs to be such that the particles do not deform and fill the capillary void spaces in the absorbent member or article to an unacceptable degree, so-called gel blocking, thereby inhibiting the rate of fluid uptake or the fluid distribution by the member or article. Once gel-blocking occurs, it can substantially impede the distribution of fluids to relatively dry zones or regions in the absorbent article and leakage from the absorbent article can take place well before the particles of absorbent polymer in the absorbent article are fully saturated or before the fluid can diffuse or wick past the “blocking” particles into the rest of the absorbent article.
Another property of these particulate superabsorbent polymers is what is called gel bed permeability. Gel permeability of particulate superabsorbent polymer composition is a measure of how rapidly liquid flows through the mass of swollen particles. In general, the gel permeability of a zone, or layer, comprising swollen particulate superabsorbent polymer can be increased by increasing the cross link density of the polymer gel, thereby increasing the gel strength. Particulate superabsorbent polymers with relatively high gel permeability can be made by increasing the level of internal crosslinking, which increases the strength of the swollen gel, but this typically also reduces the absorbent capacity of the gel undesirably, as described above.
In the past decade, significant investments have been made to improve the performance of such SAP's, e.g. to provide a higher absorbent capacity per volume, to improve fluid distribution throughout the SAP's, and to reduce so-called gel blocking of the SAP's. One area of focus has been to modify the surface of SAP particles such that optimum gel permeability is achieved without significantly compromising the absorbent capacity.
The current trend in absorbent articles including diapers, is toward ever thinner core constructions having a reduced or zero cellulose fiber, or fluff, content and an increased SAP content. As diaper cores become thinner, the SAP particles must possess properties that historically have been supplied by fluff pulp. Since reducing the fiber content between the superabsorbent polymers increases the risk of gel blocking, there is a need to provide thinner cores without much or any fibers, which do not suffer from gel blocking.
Hence, there is still a need to improve the absorbent capacity and gel strength of particulate superabsorbent polymer at the same time.